Spin-Orbit Coupling of Ln3+ Induces and Modulates Thermally Activated Delayed Fluorescence in Heterometallic LnAgP3 Clusters.

The rational design of Ag(I)-based thermally activated delayed fluorescence (TADF) materials requires fundamental understanding of structure-property relationship governing their emission characteristics. In this work, coordination with Ln3+ ions endows the resulting compounds LnAgP3 (Ln = Gd/Eu/Y) with pronounced photoluminescence. Temperature-dependent emission spectra and decay lifetime measurements reveal that Y3+ and Gd3+ incorporation induces distinct TADF activity in the [AgP3] moiety. In EuAgP3, combined the experimental results supports efficient energy transfer from [AgP3] to Eu3+ via both singlet energy transfer (SET) and triplet energy transfer (TET) pathways. Modulating the Eu:Gd molar ratio within a lattice enables precise control over the TADF performance of the [AgP3] unit. At a Eu:Gd ratio of 0.5:0.5, optimal TADF performance of the [AgP3] moiety is observed with a larger k(S1→S0) value of 1.05 × 107 s-1 and a shorter TADF decay time of 6.45 µs. Theoretical calculations further reveal that the SOC of the 4f orbitals perturbs the electronic structure of [AgP3], compressing ΔE(S1-T1) to <0.2 eV, which enables reverse intersystem crossing (RISC) and thus TADF. Consequently, varying the Eu:Gd ratio provides an indirect handle over the SOC-mediated Ag↔Eu energy-transfer pathway, offering an effective route to regulate the TADF performance of the [AgP3] moiety.